When postmen do find themselves confronted by a dog, they are told to position their bag between themselves and the animal to fend off any attack.

They are told to back away slowly, but not to turn and run, as this could trigger a chase response in the animal. Staff are also urged not to stare at the dog.

Many postal staff already carry peppermint spray, which releases a cloud of gas intended to distract and disorientate dogs for up to 20 minutes.

It can be requested by any concerned employee, but postal workers have said it is difficult to use when being attacked.

The new spray works in a different way. A Royal Mail spokesman said: "The device releases air at speed, creating a hissing noise which is designed to pull the dog up and stop its progress."

It is being trialled by staff in south-east England and could be rolled out nationwide if it produces good results.

The spokesman added: "This is something we take extremely seriously, and we give training to staff about ways to deal with dogs."

The Royal Mail is at a loss to explain the recent increase in dog attacks, but it comes at a time when animal welfare organisations have warned about the rise in the numbers of "weapon dogs", or aggressive animals used as status symbols.

Each attack on a postal worker is logged and owners of aggressive animals are contacted by Royal Mail. Injured staff are eligible for compensation under a Dog Attacks Compensation Scheme for any overtime or shift allowances which they miss as a result.

Some staff have been seriously wounded. In 2008, Keith Davies had to undergo a six-hour operation after being attacked by two rottweilers while delivering post in Cambridge.

A spokeswoman for the Communication Workers Union, said: "It is one of the biggest health and safety issues that delivery staff face. We're well aware that it got worse last year.

"It can be very traumatising for workers not knowing what is behind the garden gate or the letter box."

The union has launched the "Bite-Back" Campaign, calling for changes in the law on dangerous dogs to give postal workers greater protection while on private property.